Flexible polyurethane foams are commonly made by reacting petroleum-based polyols or polyol compositions with organic polyisocyanates in the presence of catalysts, blowing agents and other optional ingredients. Since the 1960's, flexible polyurethane foams have been used as a cushioning, loadbearing, and comfort-providing component of automotive, transportation, and other seating designs. To meet the widely varying requirements of various seat designs, it is necessary for the commercial flexible foam producers to have technology that allows them to easily and economically vary the hardness of the flexible foams they produce.
Hardness and loadbearing are two terms that the flexible foam industry often interchanges. Both terms should be understood to relate to the same, weight supporting, physical characteristic of a flexible polyurethane foam.
The ability of a flexible polyurethane foam to receive and support a weight is commonly defined as its loadbearing capacity. Quantification of this property is done in an Indentation Force Deflection (IFD) test following the guidelines of standardized procedures such as in ASTM D 3574, Test B1.
A wide variety of techniques for varying foam hardness exist. The most commonly used among these techniques involve varying the foam's density, isocyanate index, and/or polyol functionality, and the use of copolymer polyols, with copolymer polyols being the most useful. Copolymer polyols typically consist of a polyether polyol serving as a carrier liquid for millions of tiny, typically styrene/acrylonitrile-containing particles. Special additive molecules and process steps are necessary to produce a stable dispersion of the particles. In the final polyurethane foam, these added particles function as a classical filler and are a convenient way to adjust the hardness or loadbearing capability of the flexible foams.
Although copolymer polyols are capable of varying foam hardness, they exhibit a number of disadvantages. These disadvantages include variations in the weight percent of suspended solids, the viscosity of the neat product, and product color. Variations in these features lead to lot-to-lot performance differences when using copolymer polyols in a production environment. In addition, a common industrial problem with using copolymer polyols is the plugging of filters located at key points in a foam production plant. The filters are there to catch any trash arising from normal shipping and handling operations. Even though copolymer polyols are quoted as having particles designed to be in the range of 1 micron in size, it is common for any given shipment of product to plug 100 micron and even larger size filters.